Speckle is a phenomenon created with laser light sources, due to the fact that laser light is coherent. Parallels and synchronized wavefronts simultaneously hit the projection surface. When the light hits the surface, it creates constructive and destructive interference. The first category of interference induces an image deterioration that is often visible by human eye and/or by sensors. In addition to a loss of image quality, visual comfort of the viewer may also be affected.
Several techniques are used in order to remove or reduce speckle. In many cases, light coherence reduction techniques are used. For instance, the light hitting the projection surface is provided from various projection angles. Polarized laser light hitting a depolarized film is also used. Otherwise, illumination using various laser wavelengths may also be used.
Another approach consists in using vibration of the projection surface. The resulting systems are complex, expensive, and involve very specific hardware material.
WO2009/077198 describes an optical system comprising a coherent light source and optical elements for directing light from the source to a target. The optical elements include at least one diffusing element arranged to reduce a coherence volume of light from the source and a variable optical property element. A control system controls the variable optical property element such that different speckle patterns are formed over time at the target with a temporal frequency greater than a temporal resolution of an illumination sensor or an eye of an observer so that speckle contrast ratio in the observed illumination is reduced. The variable optical property element may be a deformable mirror with a vibrating thin plate or film. This solution requires modifying the projection system in order to integrate additional components, such as diffusing elements.
WO2007/112259 describes a system and method for reducing or eliminating speckle when using a coherent light source. A refracting device, comprising a birefringent material, is positioned such that the refracting device intercepts the coherent light. The refracting device rotates, thereby causing the ordinary and/or extraordinary beams to move. The human eye integrates the movement of the beams, reducing or eliminating laser speckle. The refracting device may include one or more optical devices formed of a birefringent material. Wave plates, such as a one-half wave plate, may be inserted between optical devices to cause specific patterns to be generated. Multiple optical devices having a different orientation of the horizontal component of the optical axis may also be used to generate other patterns. Furthermore, the refracting device may include an optical device having multiple sections of differing horizontal components of the optical axis. This solution involves a complex and expensive component, the rotating refracting device. Moreover, the integration of such device requires a specific global design.
US2009/0161196 describes a system and method for temporally varying the interference pattern generated by a coherent light source to homogenize the speckle pattern so that the speckle phenomenon is less observable. In accordance with an exemplary embodiment, an oscillating refractive element may be disposed within an optical system to create a temporally variable phase shift in the lights rays emanating from a coherent light source to eliminate static interference patterns on a light receiving element, reducing the speckle phenomenon. This solution involves a complex and expensive component, the oscillating refractive element. Moreover, the integration of such device requires a specific design of the projection system.
JP2001296503 describes a device for reducing speckle, which can decrease the speckle pattern appearing on the illumination face to produce uniform illumination. The laser light with linearly polarized light having the polarization plane rotated by 45 degrees is made incident on a first polarization beam splitter to be separated into the P polarized light component and S polarized light component. The P polarized light component is transmitted while the S polarized light component is reflected to equally separate the both polarized light components. The P polarized light component directly propagates while the S polarized light component is reflected by a return prism to produce an optical path difference longer than the coherence length from the optical path length of the P polarized light. Thus, incoherent laser light having two kinds of polarized light components with the optical path difference longer than the coherence length is obtained as the output from the second polarization beam splitter. This arrangement involves a complex and expensive optical system.
US2009046361 describes a reflection type screen capable of suppressing effect of external light and obtaining a wide angle of view and a forward projection system having a reflection type screen. The reflection type screen includes a transparent prism arranged in parallel to the longitudinal direction of a base. The transparent prism has a transparent plane and a diffusion-reflection plane formed by arranging a reflection film on one of the surfaces of the transparent prism. Projected light is diffused/reflected by the diffusion-reflection plane. The diffusion-reflection plane may be vibrated in order to decrease speckle noise by laser light beams. For example, by vibrating the reflection type screen in up and down, right and left, or back and fourth direction, the diffusion-reflection plane may be vibrated. Otherwise, by varying the interval W between the base and the front sheet by an air pump, the diffusion-reflection plane may also be vibrated. This arrangement involves a particular construction with a forward projection system having a complex screen configuration.
Thus, there is a need for a novel micro-projection system with reduced speckle having MEMS micro-mirrors and MEMS components in general, that do not present the above mentioned drawbacks, namely the complexity and costs problems caused by using specific configurations with additional components used only for speckle reduction.